This invention concerns the means for operating the parallel transversely-extending slats of conventional sieves and chaffers comprising the cleaning means for harvester combines, in order to open them for removal of stalks and other relatively large debris and to then return the slats to their normal operating position. Heretofore, the slats of these devices were opened and closed simultaneously, for cleaning purposes, by means of a suitable lever or handle means connected with a slotted strap or bar extending transversely of the slats and engaged with crank means formed in each of the wires or rods on which the slats are mounted for oscillating them simultaneously. Constructions of this kind are illustrated by U.S. Pat. Nos. 2,253,296, 2,262,099 and 2,413,382. The disadvantages of these prior arrangements reside mainly in that proper adjustment of the operating opening of the slats was difficult to attain and once the slats were opened, it was almost impossible to repeatedly return the slats to precisely their originally set position because of wear in the lever or handle mounting arrangement, loosening of the connections between the operating lever and the slotted strap or bar and wear in the mutually engaging portions of the adjusting strap and the cranks formed in the slat carrying rods. Efforts to overcome these difficulties culminated in the herein described invention.